Hot-electron-induced degradation of semiconductor devices is considered to be one of the most limiting factors in down-scaling transistors. Hot-electron-induced degradation is caused by the introduction of a relatively high voltage to the drain of a silicon metal-oxide-semiconductor (MOS) transistor. A widely accepted physical model is that the introduction of the voltage causes electrons to gain enough energy to break silicon-hydrogen bonds formed in the interface between the gate oxide and the silicon substrate.
If the electrons gain enough energy, the collision therebetween causes hydrogen atoms to break from the silicon and diffuse harmlessly away from the interface. The remaining silicon radicals, however, remain and become a charge trap which may undesirably shift the threshold voltage by attracting and holding electrons rather than allowing them to flow between diffused regions (a source and a drain) as required. For example, in a 1-micron transistor having an approximately 200 Angstroms thick layer of gate oxide, approximately 8 volts may be (depending upon the details of the structure) all the voltage required to cause degradation of transistor characteristics.
Several methods have been developed to attempt to resolve hot-electron-induced degradation, all of which have generally fallen short. One such method comprises the use of a lightly-doped drain (LDD) positioned proximate a highly-doped region. The LDD spreads the electric field in an attempt to prevent the hot electrons from gaining sufficient energy to break the silicon-hydrogen bonds. The use of an LDD helps reduce but does not eliminate the effects of hot-electron-induced degradation. Additionally, the use of an LDD may actually further degrade the transistor by creating higher resistance than desired.
Another method used to try to reduce the effects of hot-electron-induced degradation is the use of a nitrogen rather than a hydrogen ambient to perform the final anneal to reduce the amount of hydrogen available to bond with silicon. Although the use of a nitrogen ambient reduces the amount of hydrogen available to bond with silicon, it is difficult under present technologies to eliminate hydrogen entirely, since many processes are hydrogen-dependent. Thus, while the use of nitrogen may reduce the amount of hydrogen present, it does not eliminate hydrogen nor the problems caused by hot-electron-induced degradation.
An additional method to reduce the amount of hot-electron-induced damage is to introduce fluorine or chlorine during thermal oxidation which is disclosed in Nishioka, Dramatic Improvement of Hot-Electron-Induced Interface Degradation in MOS Structures Containing F or Cl in SiO.sub.2, IEEE Electron Device Letters, Vol. 9, No. 1 (January, 1988). Unfortunately, the use of fluorine as disclosed by Nishioka is difficult to control and typically requires a wet rinse which adds contaminants to the surface and therefore causes additional problems. Likewise, the use of chlorine requires strict controls which add to the complexity of the process and can create additional problems if the proper chlorine levels are not strictly maintained. Thus, there is a need for a relatively simple method for forming transistors which avoid the effect of hot-electron-induced degradation.